Method and apparatus for supporting the roofs in underground excavations

ABSTRACT

A walking mine roof support wherein one or more roof engaging caps are pivotable on the upper end of at least one extendable pit prop which is mounted on the floor engaging base. Balancing rams connect the rear ends of the caps with the base or such rams carry bars which are movable against the roof behind the caps. The props and the rams are operated by a hydraulic control system in response to signals from pressure gauges mounted on the front and rear ends of the caps. Such signals indicate changes in pressure which the mine roof exerts against the respective ends of the caps.

United States Patent Von Hippel 1 June 27, 1972 [54] METHOD ANDAPPARATUS FOR SUPPORTING THE ROOFS IN UNDERGROUND EXCAVATIONS [72]Inventor: Hensjoachim Von l'llppel, 4628 Linen F 2286 Muhlenkamp 12,777i Oberstenweiler, Germany [22] Filed: Sept. 8, 1970 [2i Appl. No.:70,048

[30] Foreign Application Priority Date Dec. 4, 1969 Germany ..P i9 60807.2

[52] U.S. Cl. ..61/45 D, 61/63, 91/390, 248/354 l-l, 299/1 [5 1] Int. Cl..E2ld 11/16 [58] Field ofSearch ..6l/45 D, 63; 299/1; 73/389; 9l/390;248/354 H, 356, 357

[56] References Cited UNlTED STATES PATENTS l,796,667 3/1931 Rundqvist..91/390 x Primary Examiner-Dennis L. Taylor Attorney-Michael S. Striker57 ABSTRACT A walking mine roof support wherein one or more roofengaging caps are pivotable on the upper end of at least one extendablepit prop which is mounted on the floor engaging base. Balancing rarnsconnect the rear ends of the caps with the base or such rarns carry barswhich are movable against the roof behind the caps. The props and therams are operated by a hydraulic control system in response to signalsfrom pressure gauges mounted on the front and rear ends of the caps.Such signals indicate changes in pressure which the mine roof exertsagainst the respective ends of the caps.

PATENTEnJun 27 m2 SHEET 10F 4 FIG] IN VE N TOR HANSJOACH/M VON H/PPE Lhis ATIDRNEY PATENTEDJUH? I972 SHEET 20F 4 IN VEN TOR HANSJOACHIM VONH/PPEL his ATTORNEY PATENTEnJum m2 SHEET 30F 4 F/GA IN VEN TORHANSJOACH/M VON H/PPEL PATENTEDJUHEY :272 3.672.174

saw u or 4 V IN VEN TOR HANSJOACH/M vo/v HIPPEL BY %z/4// hisATTORNEYMETHOD AND APPARATUS FOR SUPPORTING THE ROOFS IN UNDERGROUND EXCAVATIONSBACKGROUND OF THE INVENTION The present invention relates to a methodand apparatus for supporting and preventing collapse of the roofs inunderground excavations. More particularly, the invention relates toimprovements in a method and apparatus for supporting the roofs inunderground excavations wherein the material is removed from the mineface and the apparatus which supports the roof is intermittentlyadvanced toward the face to prevent cave-in in the area accommodatingthe material removing machinery and the conveyors for transport ofremoved material to the surface.

Presently known mine roof supporting apparatus for use in longwallmining normally comprise a battery of discrete or interconnected mineroof supports'each of which has a roof engaging cap, a floor engagingbase or support and one or more pit props which are mounted on the baseand carry the cap. The props can be extended and contracted to move thecap against or away from the roof and to furnish the necessaryresistance to collapse of the roof. lfthe cap is supported by a singleprop or by several transversely aligned props, it is free to tilt withreference to the prop or props so as to automatically find an optimumposition for engagement with the roof when the props are extended tourge the cap against the underside of the roof.

It is a well known fact that, during mining of coal or similarlydeposited minerals by longwall mining, the roof of the excavation tendsto fracture in the general plane of the mine face. The relatively longfront portions of the caps, namely, those portions which extendforwardly of the props, are'thereby required to resist greatly increasedloads. Such loads considerably exceed those which are resisted by therelatively short rear portions of the caps, i.e., those portions whichpoint toward the waste behind the mine roof supports. Frequently, theload upon the rear positions is insufficient to prevent the frontportions from being forced down in the area immediately in front of themine face. This contributes to furtherloosening of overlying strata andcan lead to sudden overloading of mine roof supports and to collapse ofthe roof supporting structure.

SUMMARY OF THE INVENTION An object of the invention is to provide amethod which prevents the collapse of roofs in underground excavationsand according to which the roof engaging assembly is protected againstundue bending and/or other stresses.

Another object of the invention is to provide a method according towhich the collapse of the roof in an underground excavation is preventedin a fully automatic way so that the number of attendants can be reducedand that the prevention of roof collapse is not dependent on the skilland/or conscientiousness of the attendants.

A further object of the invention is to provide a method according towhich localized stressing of the roof engaging assembly can becounteracted without any delay and which is particularly suited toneutralize overstressing of the roof engaging assembly in longwallmining wherein the roof tends to develop fissures or cracks in thegeneral plane of the mine face.

An additional object of the invention is to provide a mine roof supportwhose roof engaging assembly is protected against undue stresses in anovel and improved way.

' Still another object of the invention is to provide a mobile mine roofsupport which can be combined with similar mine roof supports to preventcollapse of a larger part of the roof in an underground excavation andto provide the thus combined apparatus with a novel control system whichautomatically responds to overstressing of any given roof engagingassembly and can at the same time adjust the adjoining assemblies toinsure more uniform distribution of stresses.

A concomitant object of the invention is to provide a mine roof supportwith novel means for monitoring the load upon its roof engaging assemblyand with novel means for automatically responding to signals from themonitoring means to avoid overstressing of the roof engaging assemblyand to also prevent collapse of the roof.

An ancillary object of the invention isto provide a mine roof supportwhich need not be shifted toward the mine face as frequently aspresently known mine roof supports.

Another object of the invention is to provide novel means for balancingthe roof engaging assembly of a walking mine roof support. 7

The method of the present invention is resorted to for preventingcollapse of the roof in an underground excavation, particularly inlongwall mining. The method comprises the steps of supporting frombelow, with a variable force, an elongated strip of the roof in front ofand substantially at right angles to the mine face in the excavation,measuring the pressure which the supported strip of the roof exerts in adownward direction, and adjusting the variable force in accordance withthe result of such measurement. The measuring step preferably comprisesmonitoring the load on the roof engaging assembly which applies thevariable force to the strip in the region of at least one end of thestrip, and the adjusting step preferably comprises automaticallyadjusting the variable force as a function of the result of measurementsat the one end of the strip. Such measurements can be carried out byresorting to pressure gauges which can be installed in the roof engagingassembly and transmit signals which are utilized for adjustment of thevariable force by way of a suitable control system, preferably by way ofa hydraulic control system which can regulate the fluid pressure in oneor more props or struts on which the roof engaging assembly is mountedand which are in turn mounted on a floor-engaging support.

In accordance with a more specific feature of the invention, thepressure which the strip exerts is measured at least in the region ofthat end of the strip which is adjacent tothe mine face, and theadjusting step comprises reducing the force at the other end of thestrip when the pressure at the first mentioned end of the strip (closeto the mine face) increases. In some instances, it mightbecome necessaryto terminate the application of the force at the other end of the stripand to increase the force which acts against the end of strip which isadjacent to the mine face. This can be achieved by connecting the rearportion of the roof engaging assembly with one or more double-actingbalancing rams which can apply a thrust or a pull whereby a thrustincreases the force which is applied to the other end of the stripwhereas a pull "causes the front portion of the assembly to exert agreater force against the strip in the region of the mine face. This isdue to the fact that the roof engaging assembly is pivotally mounted onone or more props or struts and acts not unlike a two-armed lever whosefront arm bears against the roof with a force which increases if therear arm is caused to actually move downwardly or is merely subjected tothe action of a downwardly oriented force.

In accordance with another more specific feature of the invention, theadjusting step comprises maintaining constant the force which actsagainst the end of the strip that is remote from the mine face andincreasing the force which acts against the remainder of the strip whenthe result of the measurement indicates a rise in pressure which isexerted by the strip. This merely involves preferably automaticadjustment of the prop or props whereby the force which is transmittedby the balanc ing ram or rams remains unchanged.

The roof engaging assembly is subject to bending stresses which shouldnot exceed a predetermined value. The adjusting I step may comprisechanging the force in the region of that end of the strip which isremote from the mine face when the pressure transmitted to the roofengaging assembly generates a which are adjacent to each other and tothe first mentioned strip, measuring the pressure which the additionalstrip or strips exert in a downward direction, and adjusting thevariable force which acts on one or more strips when the pressureexerted by a neighboring strip varies. This renders it possible tofurther reduce the likelihood of collapse in that the measurement bringsabout adjustment of the force which acts against the most afi'ectedstrip as well as against one or more adjacent strips.

The novel features which are considered as characteristic of theinvention are set forth in particular in the appended claims. Theimproved mine roof support itself, however, both as to its constructionand its mode of operation, together with additional features andadvantages thereof, will be best understood upon perusal of thefollowing detailed description of certain specific embodiments withreference to the accompanying drawing.

BRIEF DESCRIPTION OF TI-IE DRAWING FIG. 1 is a side elevational view ofa mine roof support which embodies one form of the invention and islocated in an underground excavation in front of the mine face;

FIG. 2 is an end elevational view as seen from the right-hand side ofFIG. 1 and illustrates three groups of pairwise arranged mine roofsupports each of which is constructed in a manner as shown in FIG. 1;

FIG. 3 is a plan view of the structure shown in FIG. 2;

FIG. 4 illustrates the hydraulic control system of the mine roof supportshown in FIG. 1;

FIG. 5 is a schematic side elevational view of a modified mine roofsupport;

FIG. 6 is a plan view of the mine roof support shown in FIG.

FIG. 7 is a fragmentary side elevational view of a third mine roofsupport; and

FIG. 8 is a plan view of the third mine roof support.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring first to FIG. 1,there is shown a so-called walking mobile mine roof support whichcomprises a roof-engaging assembly having a cap or roofbar 1, afloor-engaging base member or support 2, and a propping device hereshown as consisting of a single adjustable hydraulic pit prop or strut 3which carries the cap 1 and is of variable length. The upper end portionof the prop 3 engages an intermediate portion of the cap 1. The rearportion 4 of the cap 1 (behind the prop 3) is connected to the rearportion of the support 2 by an adjustable balancing device here shown asconsisting of a single ram 5 which preferably constitutes ahydraulically operated prop or strut of variable length and is designedto exert on the rear portion 4 a thrust or a pull, depending on thenature and distribution of pressures on the cap 1 and in line with thedirection of such pressures. In the illustrated embodiment, thebalancing ram 5 comprises a double-acting cylinder which is mounted onthe support 2 and a piston whose piston rod is coupled to the rearportion 4 of the cap 1. For better guidance of the piston rod, thelatter can be slidably telescoped into an upwardly projecting tubularextension of the double-acting cylinder as shown in FIG. 1. The prop 3may also comprise a double-acting cylinder which is mounted on thesupport 2 and a piston whose piston rod is coupled to an intermediatepor tion of the cap 1. If desired, the prop 3 can employ a singleactingcylinder which can lift the piston rod and the cap 1 in response toadmission of a pressurized hydraulic fluid into its chamber whereby thecap can descend by gravity and/or in response to the pressure of themine roof when the pressure in the chamber of the cylinder is reduced.It is also possible to replace the single prop 3 with a group of two ormore props. Embodiments with plural props are shown in FIGS. 5 to 8.

The mine face in an underground excavation wherein the mine roof supportof the present invention can be put to use is shown at 50. The mine roofsupport can be used with a battery of similar or identical mine roofsupports which can be assembled and grouped side-by-side in a manner asshown in FIGS. 2 and 3. When the material along the mine face 50 isremoved by the customary machinery which forms no part of the presentinvention, some or all of the mine roof supports are advanced toward themine face, either singly or in groups of two or more, and the removal ofmaterial from the mine face begins anew. The thus removed material istransported from the excavation by a system of conveyors one of whichruns along the mine floor in front of the mine face 50 and is shown inFIG. 1, as at 150. The conveyor can be coupled to the mine roof supportsso that it is automatically advanced when the mine roof supports arecaused to walk in a direction to the fight, as viewed in FIG. 1. Themeans for advancing the mine roof supports toward the mine face is ofknown design and, therefore, is not shown in the drawing. Such advancingmeans normally comprises hydraulically operated cylinder and pistonassemblies.

In the illustrated mine roof support, the length of the front portion ofthe cap 1 (in front of the prop 3) exceeds the length of the rearportion 4. The upper side 6 of the rear portion 4 slopes downwardly andrearwardly. An intermediate portion of the cap 1 contains a block orheadpiece 7 which is articulately connected with the piston rod of theprop 3 by a joint here shown as including a transverse pivot pin 8. Thecap 1 rests on the piston rod of the prop 3 not unlike a balance beam orweighbeam. However, it is equally within the purview of the invention toprovide the block 7 with a concave socket for a spherical head of thepiston rod. It is preferred to design the cap 1 in such a way that itsmaximum width is in the region of the pivot pin 8. Also, the portion ofmaximum height of the cap 1 is preferably in the region of the block 7and pivot pin 8.

The support 2 is provided with side walls 9 and a rear wall 10 whichacts as a shield to retain loose rock 11 of waste material againstpenetration into the space between the side walls 9. The rear wall 10preferably extends upwardly to or beyond the level of the upper end ofthe cylinder of the prop 3. The upper edge portion of the rear wall 10carries a hinge 12 for the lower rear end of a radius rod or link 13 thefront end of which is coupled to the cap 1 in front of the pivot pin 8,preferably by means of a further pivot pin 14. The link 13 slopesdownwardly and rearwardly from the pivot pin 14 toward the hinge 12. Thepurpose of the link 13 is to stabilize the position of the cap 1 but itdoes not interfere with necessary pivotal movements of the cap withreference to the prop 3 and balancing ram 5 or with expansion andcontraction of the part 3 and/or 5. The rear end portion 15 of the link13 is wider than the remaining major part of the link. A hydraulicadjusting unit or control unit 25 is mounted on the forward part of thesupport 2 in front of the prop 3 and is connected with various othercomponents of the hydraulic control system by pipes and conduits whichin part extend in or along the link 13. FIG. 1 shows conduits 24 and 32which respectively connect the adjusting unit 25 with signal generatingpressure gauges or load monitoring members 22 and 31. The link 13shields the adjacent or surrounded portions of the conduits 24 and 32.

FIGS. 2 and 3 show that two or more adjoining mine roof supports can beassembled to form a group constituting a twin or multiplex mine roofsupport. The means for coupling the adjoining mine roof supports maycomprise bolts or analogous fasteners which rigidly connect the sidewalls 9 on the supports of neighboring mine roof supports. Each groupof, for example, two mine roof supports may share a common link 13 whichis then located in the central vertical plane extending between twointerconnected mine roof supports. The pintle of the hinge l2 and thepivot pin 14 for such common link 13 may extend through all of theinterconnected caps 1 to enhance the rigidity of the apparatus,especially as concerns the resistance to lateral distortion.

Referring again to FIG. 1, the balancing ram 5 has a rounded lower endportion which is received in a bearing 16 of the support 2. A pivot pin17 connects the ram 5 to the support. The upper end of the ram isarticulately connected to the rearmost part of the rear portion 4 of thecap 1 by a pivot pin 18. The support 2 is further provided with aconcave socket or shoe 19 forthe spherical lower end portion of the prop3. The socket 19 is located between the adjusting unit 25 and thebearing 16.

The aforementioned pressure gauge or monitoring member 22 is mounted inthe foremost part 20 of the cap 1 below an elastically deformable coverplate 23 consisting of steel or the like. The cover plate 23 abutsagainst the underside of the mine roof 21 and the purpose of thepressure gauge 22 is to monitor the pressure which the mine roof 21exerts against the cover plate 23 in the region of the front portion 20of the cap 1. The pressure gauge 22 produces signals which are utilizedfor automatic adjustment of the prop 3 and/or balance ram 5 as afunction of the pressure against the cover plate 23 close to the mineface 50. As best shown in FIG. 4, the pressure gauge 22 comprises acylinder 22a whose chamber below a reciprocable piston 22!: is filledwith hydraulic fluid and wherein the piston 22b is biased upwardly by aspring 22c. A

motion transmitting plunger 22d is connected with the piston 22b andbears against the underside of the cover plate 23. The aforementionedconduit 24 connects the chamber of the cylinder 22a with the adjustingunit 25. The latter contains all the necessary hydraulic accessoryequipment some of which is shown in FIG. 4 and which includespressure-responsive valves, relief valves, check valves, control valvesand others. The construction of many valves in the adjusting unit 25 isknown and, therefore, some such valves were omitted for the sake ofclarity and the others are merely shown by appropriate symbols.

Conduits 26, 28 connect the adjusting unit 25 with the chambers in thecylinder of the balance ram 5 and a further conduit 27 connects theadjusting unit 25 with the chamber below the piston in the cylinder ofthe prop 3. A supply conduit 29 connects the adjusting unit 25 with asource 30 of pressurized hydraulic fluid. As shown in FIG. 3, thissource extends along the front portions of the supports 2 of the batteryof mine roof supports which are located in front of the mine face 50.

The pressure gauge 31 constitutes a second signal generating monitoringmember and is mounted in the rearmost part of the rear portion 4 of thecap 1. The construction of the pressure gauge .31 is preferablyidentical with that of the gauge 22 (see FIG. 4) and its plunger 31dabuts against a resilient cover plate 33 which overlies the rear portion4 and bears against the adjacent loose rock. The conduit 32 connects theadjusting unit 25 with the chamber in the cylinder 31a of the pressuregauge 31. The cover'plates 23 and 33 are mounted in such a way that theycan flex downwardly when the pressure against their upper sidesincreases whereby the cover plates respectively depress the plungers22d, 31d to generate pressure pulses or signals which are transmitted tothe adjusting unit 25.

The hydraulic control system of the mine roof support shown in FIG. 1includes the aforementioned adjusting unit 25, the pressure gauges 22,31, the conduits 24, 26, 27, 28, 29, 32, and the source 30 ofpressurized hydraulic fluid. This control system serves to automaticallyadjust the prop 3 and/or the balancing ram 5 in response to changes inpressure upon the cover plates 23, 33. As shown in FIG. 4, the conduit24 connects the chamber in the cylinder 22a of the pressure gauge 22with a valve 34 which controls the flow of hydraulic fluid into theconduit 27 and hence into the chamber of the prop 3. The valve 34 isconnected with the aforementioned supply conduit 29 and with a secondsupply conduit 35 which can admit highly pressurized hydraulic fluid.Such pressurization of hydraulic fluid for admission into the supplyconduit 35 can take place in a suitable pressure accumulator or thelike, not shown. When the supply conduit 35 is permitted to communicatewith the conduit 27 (by way of the valve 34), the chamber of thecylinder in the prop 3 receives fluid at a high pressure so that theprop 3 can withstand substantial pressures which are applied against theupper side of the cap 1. The

supply conduit 29 conveys hydraulic fluid at normal system pressure;this conduit contains a check valve 29a and a shutoff valve 29b.

The conduit 24 is further connected to a valve group 36 which includes apressure reducing valve 46, a pilot piston 37 and a flow interrupting orstop valve 38. The purpose of the valve group 36 is to regulate the flowof hydraulic fluid to and from the chambers of the cylinder in thebalancing ram 5. When the pressure of fluid in the conduit 24 rises,i.e., when the cover plate 23 moves the plunger 22d of the pressuregauge 22 downwardly, as viewed in FIG. 4, the fluid moves the pilotpiston 37 upwardly against the opposition of a helical spring 37awhereby the piston 37 first completes an idle stroke at before it canengage and open the stop valve 38. As the pilot piston 37 moves upwardlyto stress the spring 37a, it relieves a second spring 41 which biasesthe valve member of the pressure reducing valve 46 to closed position inwhich the valve member engages a seat in the body of the valve 46. Thepiston 37 completes the idle stroke x when the valve 46 permits thepressure in the chambers of the cylinder in the ram 5 to drop to zero.When the stop valve 38 opens, it permits hydraulic fluid to flow fromthe supply conduit 29 to a conduit 39 which is connected to an inlet ofa control valve or stop valve 40. If the valve 40 opens, the fluid canflow from the conduit 39 into the conduit 28 and hence into the upperchamber of the cylinder in the ram 5. This causes the piston rod of theram 5 to descend or at least to exert a pull on the rear portion 4 ofthe cap 1.

The control valve 40 is connected with a second control valve 42 bymeans of a conduit 43 which is further connected to the conduit 32 forthe cylinder chamber of the pressure gauge 31. A third control valve 44is also connected with the conduit 43 and with a return conduit 45 whichcan discharge fluid into an exhaust pipe 47 in response to opening ofthe pressure reducing valve 46. The valve 40 functions as a stop valvefor the conduit 39. A conduit 48 connects the control valve 42 with theconduit 26 and hence with the lower chamber in the cylinder of thebalancing ram 5. This lower chamber can receive hydraulic fluid from thesupply conduit 29 by way of an adjustable shutoff valve 290 before theautomatic control of the operation begins. Thus, the operator can decideto extend the ram 5 in response to partial or complete opening of theshutofi valve 290. It is clear that the cylinders of the prep 3 and ram5 are provided with customary pressure relief valves and exhaust valvesfor evacuation of spent fluid. Such valves are not shown for the sake ofclarity. Furthermore, even though FIG. 4 shows pilot pistonsof equalsize, it is evident that the diameters of such pistons can vary within awide'range. Also, the springs shown in FIG. 4' are preferably adjustableso that the control system can be programmed to effect opening and/orclosing of its valves in a desired sequence.

The operation of the hydraulic control system for the mine roof supportof FIG. 1 is as follows:

If a fissure 49 (shown in FIG. 1) develops in the mine roof 21 close toor in the general plane of the mine face 50, this results in theexertion of a greater pressure on the cover plate 23 for the frontportion of the cap 1 because the mine roof 21 ceases to adhere to thematerial above the mine face. The pressure gauge 22 responds to suchincrease in pressure (i.e., to downward movement of the cover plate 23)and the pressure in the conduit 24 (FIG. 4) rises. Such rise in pressureconstitutes a signal which is transmitted to the valve 34 and valvegroup 36. If the valve 34 is first to respond to such signal, itconnects the supply conduit 35 for highly pressurized hydraulic fluidwith the conduit 27 which admits fluid into the chamber of the cylinderin the prep 3. The chamber in this cylinder can receive fluid at such apressure that the prop 3 can withstand maximum permissible stresses onthe cap 1, i.e., stresses which are below those causing theaforementioned pressure relief valves for the prop 3 to open. At thesame time, or with an appropriate delay, the pilot piston 37 responds toincreasing pressure in the conduit 24 and begins to rise to relieve thespring 41 and to reduce the pressure in the cylinder chambers of thebalancing ram 5. When the pilot piston 37 completes the idle stroke x,it opens the stop valve 38 so that the latter connects the supplyconduit 29 with the conduit 39. The valve 40 permits the fluid to flowfrom the conduit 39 into the conduit 28 and thence into the upperchamber of the cylinder of the ram 5. The latter exerts on the rearportion 4 of the cap 1 a pull whereby the cover plate 23 exerts acorrespondingly increasing force against the underside of the mine roof21. The pressure of fluid in the upper chamber in the cylinder of theram can be intensified or reduced by resorting to conventional pressureregulating means, not shown.

The purpose of the control valves 40, 42 and 44 is as follows: Thesevalves cooperate with the pressure gauge 31 on the rear portion 4 of thecap 1. If the pressure exerted by the mine roof 21 on the cover plate 33for the rear portion 4 rises beyond a certain value, the pressure in thechamber of the cylinder 31a increases and the conduit 32 transmits apressure pulse to the conduit 43. The control valve 40 is first to closein response to. such pulse whereby its pilot piston permits the fluid toflow from the conduit 28 into a relief conduit 28a containing a checkvalve 28b. This terminates the pull on the rear portion 4 by way of theram 5.

The control valve 44 is next to close and it thereby shuts 05 the returnconduit 45. The control valve 42 thereupon opens and admits hydraulicfluid from the supply conduit 29 through the conduits 48 and 26 into thelower chamber of the cylinder in the ram 5. The ram 5 applies a thrustto the rear portion 4 of the cap 1 and thus resists the temporaryoverload to prevent destruction of the cap.

If the loose rock above the cover plate 33 slides away or if a cavity151 (indicated in FIG. 1 .by phantom lines) develops above the rearportion 4 for any other reason, the pressure gauge 31 responds andinitiates appropriate adjustments of the control valves 40,42 and 44.The aforedescribed controlling action repeats itself and the upperchamber in the cylinder of the ram 5 is connected with the source ofpressurized hydraulic fluid so as to apply a pull to the rear portion 4of the cap 1.

When two or more mine roof supports are mounted side-byside as shown inFIGS. 2 and 3, it is often desirable to interconnect thepressure gauges22, 31 and/or the adjusting units 25 of adjoining mine roof supports byresorting to suitable connecting conduits so that signals produced byany one of the pressure gauges are transmitted to the adjusting units ofthe adjoining mine roof supports. This enables the group ofinterconnected mine roof supports to more effectively resist pressureswhich arise when a fissure develops in the mine roof 21. For example, ifa fissure develops in a portion of the mine roof 21 above a single mineroof support, the pressure of fluid in the neighboring mine roofsupports can be raised to enable such neighboring mine roof supports toassist the most affected apparatus in bearing the increased load. Also,such raising of fluid pressure in neighboring apparatus can be effectedas a purely precautionary measure.

An important advantage of the improved mine roof support is that theroof engaging assembly (including the cap 1, its block 7 and the coverplates 23, 33) is less likely to yield to unbalanced pressure of themine roof 21. The roof engaging assembly is mounted on the prop 3 notunlike a weighbeam or balance beam so that it can pivot about the axisof the pin 8. Its angle of tilt can be positively controlled by thebalancing ram 5 which can make allowance for differential loads actingon the cap 1. For example, when the front portion (cover plate 23)experiences a rising load by that portion of the mine roof 21 which isadjacent to the mine face 50, a downwardly directed force can be appliedto the rear portion 4 by means of the balancing ram 5 to thereby impartto the front portion of the cap an upward thrust and to thus preventcontinued loosening and breakup of the strata of rock above the coverplate 23. This insures that the rock cannot penetrate into the space 152(FIG. 1) between the prop 3 and the mine face 50; such space is neededfor the material removing machinery and the conveyor 150.

As explained above, the optimum position of inclination of the cap 1 caneven be maintained if a cavity (151) develops in the roof 21 above therear portion 4 and its cover plate 33. The balancing ram 5 then againapplies a downwardly oriented force (pull) upon the rear portion 4 toinsure that the front portion and its cover plate 23 can resist the loadin the region of the mine face 50.

If the rear portion 4 of the cap 1 is suddenly subjected to a highlyincreased pressure, for example, by a mass of loosened rock (such as therock filling the cavity 151 shown in FIG. 1 the balancing ram 5 isadjusted to apply to the rear portion 4 an upwardly oriented force(thrust) to thereby resist a sufiiciently large portion of the overallpressure on the cap 1. Also, such lengthening of the balancing beam 5prevents the generation of excessive bending stresses upon the rearportion 4, namely, such bending stresses which would tend to break thecap 1 in the region of the pivot pin 8. Though the adjustments of thebalancing ram 5 could be carried out by hand, for example, byperiodically checking the pressure gauge 31 and by thereupon adjustingthe pressure in the upper or lower cylinder chamber of the ram 5 inaccordance withthe readings furnished by the gauge 31, this wouldnecessitate practically uninterrupted supervision of pressure gauges fora battery containing a large number of mine roof supports with attendantexpenditures for additional personnel. Moreover, the adjustments of thebalancing ram 5 would depend entirely on the skill, oonscientiousnessand alertness of operators.

All such problems can be avoided by employing the aforedescribedautomatic control system which is preferably hydraulic but which canalso include at least some mechanical, pneumatic and/or electrical andelectronic components. An automatic control system reacts more rapidlythan a manually controlled system and it is more reliable, especially ifthe pressures upon the cap 1 are monitored at two or more points. As arule, it is preferred to employ at least one monitoring member (22) inthe region of the front portion of the cap 1; the other monitoringmember (31) or members are optional even though they are desirable,especially in the region of the rear cover plate 33. The control systemgenerates countervailing forces which are applied to the cap 1 by way ofthe prop 3 and/or balancing ram 5 and which can be localized in such away that excessive pressures on the front or rear portion of the cap canbe compensated for by applying properly oriented forces to the sameportion and/or to another portion of the cap. This insures that the cap1 is always held in an optimum position to resist the loads due to thepressure which the elongated strip of the mine roof 21 above the mineroof support applies against the cover plates 23, 33 and/or against themedian portion of the cap.

In some instances, the control system can be designed in such a way thatthe changes in pressure detected by the monitoring member 22 are usedonly for automatic adjustment of the balancing ram 5. However, a muchgreater versatility can be achieved if the control system serves toadjust the balancing ram 5 and the prop 3, preferably in such a way thatdifferential pressures are applied to the ram and to the prop whendifi'erential loads are applied to different parts of the cap.

If the mine roof support is equipped with two monitoring members, forexample, with the pressure gauges 22, 31 which are mounted in a manneras shown in FIG. 1, the two gauges can efiect automatic adjustments ofthe prop 3 and/or balancing beam 5. Such pressure gauges, mounted at thefront and rear ends of the cap 1, render it possible to immediatelydetect a differential in the pressures acting on the front and rearportions of the cap and to immediately adjust the balancing ram 5 sothat the latter exerts a thrust or a pull.

In the control system of FIG. 4, the parts which are connected with therear pressure gauge 31 form a safety circuit which can bring about atleast temporary changes in pressurization or a reversal in the directionof action of the balancing ram 5 when the admissible bending stresses onthe cap 1 are exceeded. This control circuit can be associated with asuitable alarm (not shown) which produces visible and/or audible signalsto warn the operators of the possible collapse of the mine roof support.In the absence of excessive loads, the control system of FIG. 4 protectsthe mine roof support against destruction by temporary overloads andenables the apparatus to absorb the momentary overload resiliently.

It was found that the improved mine roof support reduces the likelihoodof loosening of roof strata, particularly in the region adjacent to themine face. The roof is gently controlled by the caps of cooperating mineroof supports and maintains its existing cohesion. This throws less loadupon the caps and protects them from undue stresses. Thus, the danger ofroof collapse in longwall mining is reduced and the likelihood oflengthy interruptions in material removal is avoided.

As stated above, the balancing ram preferably comprises a double-actinghydraulic cylinder and piston unit. However, it is equally within thepurview of the invention to employ multistage telescopic balancing ramsor pairs of single-acting rams one of which can pull and the other ofwhich can push the rear portion 4 of the cap 1.

The cover plates 23, 33 protect the respective pressure gauges 22, 31.Moreover, such cover plates insure that the respective pressure gaugescan generate signals which represent the distribution of pressures uponrelatively large portions of the roof engaging assembly. In other words,the pressure gauges need not respond to highly localized changes inpressure which could cause faulty adjustments of the prop 3 and/orbalancing ram 5.

As shown in FIG. 1, the inclination of the balancing ram 5 in such thatits axis is substantially normal to the general plane of the upper side6 of the rear portion 4 (cover plate 33). A cap whose rear portionslopes rearwardly and downwardly can act as an effective shield orbafile which prevents penetration of loose rock into the space aroundthe balancing ram 5 and prop 3. Also, any particles of loose rock arefree to slide along the inclined cover plate 33 and to accumulate on thefloor behind the rear wall of the support 2. The inclined balance beam 5insures that the thrust or pull upon the rear portion 4 is applied inline with the pressures acting on the cover plate 33. The parts 5 and 4are well in front of the region where the roof is allowed to cave inbehind the mine roof support.

The mounting of the lower end of the link 13 on the rear wall 10 of thesupport 2 substantially midwaybetween the upper side of the cap 1 andthe underside of the support 2 enhances the stability of the mine roofsupport. It was found that such stability is not affected by the factthat the ends of the balancing ram 5 and prop 3 are articulatelyconnected to the cap 1 and support 2. Additional stability is achievedbecause the upper end of the link 13 is connected to the cap 1 forwardlyof the pivot 8 for the prop 3. The conduits which are mounted in or onthe link 13 are protected at all times, regardless of the thickness ofthe seam (i.e., irrespective of the height of the excavation).

FIGS. 5 and 6 illustrate a second mine roof support wherein the basemember or support 2a carries a propping device consisting of twohydraulic pit props or struts 3a which are inclined toward each otherand form a composite prop for an elongated head piece or beam 7a of theroof engaging assembly. This beam is urged against the mine roof 21 andis provided with longitudinally extending guides or ways 51 in the formof grooves or the like. The beam 7a is flanked by two roofbars or capsla which also form part of the roof engaging assembly and have limitedfreedom of pivotal movement about transverse horizontal pivot pins 8a.The pins 8a carry roller followers which can travel along the respectiveguides 51 to move the caps 1a forwardly or rearwardly, i.e., toward oraway from the mine face 50. (as indicated by the doubleheaded arrowshown in FIG. 5). The front end portions of the caps la are connected toeach other by a cover plate 23a the central portion of which overlies apressure gauge 22. The rear portions 4a of the caps 1a support abalancing device having two discrete balancing rams 5a each including apiston rod 52 which carries a short bar 53 movable against or away fromthe mine roof 21. The conduits which connect the pressure gauge 22,props 3a and rams 5a to the adjusting unit 25a are not shown in FIGS. 5and 6. The adjusting unit 250 can automatically adjust the props 3aand/or the balancing rams 50 substantially in the same way described inconnection with FIG. 4. The pressure gauge 22 again responds to flexingof the resiliently mounted cover plate 23a which overlies theinterconnected front end portions of the caps la. Due to the forklikeconstruction of the composite cap including the caps la, a singlepressure gauge 22 suffices to produce the necessary signals. If thepressure upon the cover plate 230 increases, the balancing rams 5a arecaused to move their piston rods 52 upwardly and to exert the necessarycounterpressure.

As shown in FIG. 5 by broken lines, the cylinders of the balancing rams5a can be articulately connected to the rear wall of the support 2a.This enables these rams to exert a thrust as well as a pull tocompensate for changes in pressure upon the front portions of the caps1a.

A double-acting shifting cylinder 56 is pivotably mounted in the beam 7aand its piston rod is articulately connected to the caps 1a so that thecaps can be moved lengthwise relative to the beam. The arrangement canbe such that, when the mine roof support is to be advanced toward themine face 50, the caps la are advanced in a first step and the beam 7ais ad vanced in the next, step.

The mine roof support of FIGS. 5 and 6 can be grouped with similarapparatus in such a way that the caps In of adjacent apparatus arestaggered in a direction at right angles to the mine face. In this way,the caps can support a larger area of the roof 21. Moreover, the caps lacan be moved forwardly toward the mine face 50 while the respectivesupports 2a remain at a standstill. This renders it possible to increasethe intervals between successive advances of the supports 2a. Since thecaps 1a are pivoted to the beam 7a and to the piston rod of the shiftingcylinder 56, they can assume positions of optimum inclination withreference to the roof 21. The balancing rams 54 (as shown by solidlines) are mounted on the rear portions 4a of the respective caps la andare designed to produce a thrust by pushing the corresponding bars 53against the roof 21. If they are coupled to the support 2a, the rams 5acan exert a thrust ora pull, preferably in automatic response to signalsgenerated by pressure gauges (not shown) on the rear portions 40 and/orin response to signals from the pressure gauge 22. When the bars 53 arelocated below a cavity, a pull produced by balancing rams 5a connectedto the support 2a enables the front portions of the caps 1a to urge thecover plate 230 against the roof 21 with a much greater force. This doesnot afiect the movability of caps 1a along the beam 70. The bars 53slide along the roof 21 and protect the balancing rams 5a when the mineroof support is advanced toward the mine face 50. The bars 53 thenconstitute shoes which move behind the beam 7a.

Referring finally to FIGS. 7 and 8, there is shown a mine roof supportwherein a single short bar 53b is articulately connected to a single cap1b by one or two transversely extending pivot pins 54 which carry rollerfollowers 55 arranged to travel in guides or ways 51b provided in theadjacent beams 7b. Thus, the bar 53 can be entrained by the cap 1b whenthe latter is caused to advance toward the mine face 50. The beams 7bflank the single cap lb and their front portions contain shifiingcylinders 56b whose piston rods are connected to the cover plate 23bwhich is mounted on the front end portion of the cap lb. The pressuregauge 22 is mounted below the central portion of the cover plate 23a. Asingle balancing ram 5b is mounted on the rear portion 4b of the cap 1band has a piston rod 52b which can move the bar 53b upwardly. The propsare shown at 3 b.

When the cap lb is caused to move toward the mine face (either with orrelative to the beams 7b), the bar 53b shares all movements of the capbut is pivotable about the pin 54. Thus, the bar 53b can move in avertical plane but always trails the cap lb. In the embodiment of FIGS.7 and 8, the cylinders 56b can shift the cap lb with reference to thebeams 7b or vice versa. A single balancing ram 5b sufiices here becausethe apparatus comprises a roof engaging assembly having a single cap. Itis clear that the lower end portion of the ram 5b can be coupled. to thesupport (not shown) in the same way as shown in FIG. 1 or FIG. 5 (bybroken lines). Also, each of the mine roof supports shown in FIGS. 5 to8 can employ one or more links corresponding to the link 13 of FIG. 1.

Without further analysis, the foregoing will so fully reveal the gist ofthe present invention that others can, by applying current knowledge,readily adapt it for various applications without omitting featureswhich fairly constitute essential characteristics of the generic andspecific aspects of my contribution to the art and, therefore, suchadaptations should and are intended to be comprehended within themeaning and range of equivalence of the claims.

What is claimed as new and desired to be protected by Letters Patent is:

1. A method of preventing collapse of the roof in an undergroundexcavation, particularly in longwall mining, comprising the steps ofsupporting from below, with a variable force, an elongated strip of theroof in front of and substantially at right angles to the mine face inthe excavation; measuring the pressure which the supported strip of theroof exerts in a downward direction; and adjusting the variable force inaccordance with the result of such measurement.

2. A method as defined in claim 1, wherein said measuring step comprisesmonitoring the pressure in the region of at least one end of the stripand said adjusting step comprises automatically adjusting the variableforce in accordance with the result of measurement at said one end ofthe strip.

3. A method as defined in claim 1, wherein said measuring step comprisesmonitoring the pressure at least in the region of that end of the stripwhich is adjacent to the mine face and said adjusting step comprisesreducing the force at the other end of the strip when the pressure atthe first mentioned end of the strip increases.

4. A method as defined in claim 3, further comprising the step ofterminating the application of force to the other end of the strip andincreasing the force which acts against the first mentioned end of thestrip.

5. A method as defined in claim 1, wherein said adjusting step comprisesmaintaining the force acting against the end of the strip which isremote from the mine face at a constant value and increasing the forceacting against the remainder of the strip when the result of themeasurement indicates an increase in the pressure exerted by the strip.

6. A method as defined in claim 1, wherein the variable force istransmitted by way of a roof engaging assembly which is subject tobending stresses and wherein said adjusting step comprises changing theforce in the region of that end of the strip which is remote from themine face when the pressure transmitted to the roof engaging assemblygenerates a bending stress which exceeds a predetermined value.

7. A method as defined in claim 1, further comprising the step ofsupporting from below with a variable force at least one additionalstrip of the roof adjacent to the first mentioned strip, measuring thepressure which the additional strip exerts in a downward direction, andadjusting the variable force which acts on said first mentioned strip inaccordance with the result of measurement of the pressure transmitted bythe additional strip.

8. A mobile mine roof supporting apparatus, particularly for use inlongwall mining, comprising a support arranged to rest on the floor ofan underground excavation; an elongated roof engaging assembly havingfront and rear portions and arranged to engage the roof in theexcavation; at least one adjustable variable-length propping devicemounted on said support and connected with said assembly between saidrear and front portions whereby the assembly is respectively urgedagainst and moves away from the roof in response to lengthening andshortening of said device; at least one adjustable balancing deviceconnected with and operative to apply to the rear portion of saidassembly a force in line with the load upon suid rcur portion; and ucontrol system comprising at least one signal generating load monitoringmember responsive to changes in load upon said assembly and adjustingmeans for adjusting at least one of said devices in response to signalsfrom said monitoring member.

9. Apparatus as defined in claim 8, wherein said balancing device isconnected with said support and is adjustable to exert a thrust or apull on the rear portion of said assembly.

10. Apparatus as defined in claim 9, wherein said balancing devicecomprises at least one hydraulically operated ram having a first portioncoupled to the rear portion of said assembly and a second portioncoupled to said support behind said propping device, one of saidportions of said ram being movable lengthwise of the ram relative to theother portion of said ram and said adjusting means comprising means formoving said one portion of said ram in response to said signals.

11. Apparatus as defined in claim 8, wherein said monitoring membercomprises a pressure gauge mounted in the front portion of said assemblyso as to be subjected to the pressure of the roof and conduit means fortransmitting signals from said gauge to said adjusting means.

12. Apparatus as defined in claim 8, wherein said monitoring membercomprises a pressure gauge mounted on the rear portion of said assemblyso as to be subjected to the pressure of the roof and conduit means fortransmitting signals from said gauge to said adjusting means.

13. Apparatus as defined in claim 12, wherein said one device is saidbalancing device and wherein said signals from said presure gauge areindicative of maximum permissible bending stresses upon said assembly.

14. Apparatus as defined in claim 8, wherein said balancing devicecomprises at least one hydraulically operated cylinder and piston unitof variable length and wherein said one device is said balancing device.

15. Apparatus as defined in claim 14, wherein the cylinder of said unitis a double-acting cylinder.

16. Apparatus as defined in claim 14, wherein said unit is a multi-stagecylinder and piston unit.

17. Apparatus as defined in claim 8, wherein the front portion of saidassembly comprises a cover plate which yieldably engages the roof andsaid monitoring member comprises a pressure gauge mounted in said frontportion below said cover plate to produce signals in response todisplacement of the cover plate by the roof.

18. Apparatus as defined in claim 8, wherein the rear portion of saidassembly has a downwardly and rearwardly inclined roof-engaging upperside and said balancing device comprises at least one hydraulicallyoperated ram coupled to said rear portion and to said support andextending substantially at right angles to said upper side.

19. Apparatus as defined in claim 8, further comprising means forpivotally connecting said assembly to the upper end of said proppingdevice.

20. Apparatus as defined in claim 8, further comprising a link connectedto said support behind said devices and to the front portion of saidassembly.

21. Apparatus as defined in claim 20, wherein said support comprises atransversely extending rear wall having an upper portion locatedsubstantially midway between the upper side of said assembly and theunderside of said support, and means articulately connecting the ends ofsaid link to said front and upper portions.

22. Apparatus as defined in claim 20, further comprising a second mineroof supporting apparatus having a floor-engaging support laterallyadjacent to and connected with said first mentioned support said linkbeing disposed in a vertical plane between said two apparatus and beingfurther connected to the front portion of the roof engaging assembly ofsaid second apparatus.

23. Apparatus as defined in claim 8, wherein said roof engaging assemblycomprises an elongated beam, means for pivotally connecting said beam tothe upper end of said propping device, and at least one roof engagingcap reciprocably supported by said beam.

24. Apparatus as defined in claim 23, wherein said cap is ivotable in avertical plane with reference to said beam.

25. Apparatus as defined in claim 23, wherein said balancing devicecomprises at least one ram mounted on the rear portion of said cap andhaving a portion movable upwardly against the roof.

26. Apparatus as defined in claim 25, further comprising means forcoupling said ram to said support.

27. Apparatus as defined in claim 25, wherein the portion of said ram isshorter than said cap and is slidable along the roof.

28. Ap aratus as defined in claim 25, further comprising means forarticulately connecting said portion of said ram to said cap.

29. Apparatus as defined in claim 8, wherein said roof engaging assemblycomprises an elongated beam connected to the upper end of said proppingdevice and a pair of interconnected elongated roof engaging capsreciprocably by said beam.

30. Apparatus as defined in claim 29, wherein said caps have front endportions connected to each other and wherein said monitoring member issupported by the front end portions of said caps.

31. Apparatus as defined in claim 29, wherein said balancing devicecomprises two hydraulically operated rams each connected with the rearportion of one of said caps.

32. Apparatus as defined in claim 8, wherein said roof engaging assemblycomprises a pair of parallel elongated beams mounted on the upper end ofsaid propping device and an elongated roof engaging cap disposed betweenand reciprocably supported by said beams.

33. Apparatus as defined in claim 32, wherein said cap is pivotable in avertical plane with reference to said beams.

34. Apparatus as defined in claim 32, wherein said balancing devicecomprises a hydraulically operated ram connected with the rear portionof said cap.

1. A method of preventing collapse of the roof in an underground excavation, particularly in longwall mining, comprising the steps of supporting from below, with a variable force, an elongated strip of the roof in front of and substantially at right angles to the mine face in the excavation; measuring the pressure which the supported strip of the roof exerts in a downward direction; and adjusting the variable force in accordance with the result of such measurement.
 2. A method as defined in claim 1, wherein said measuring step comprises monitoring the pressure in the region of at least one end of the strip and said adjusting step comprises automatically adjusting the variable force in accordance with the result of measurement at said one end of the strip.
 3. A method as defined in claim 1, wherein said measuring step comprises monitoring the pressure at least in the region of that end of the strip which is adjacent to the mine face and said adjusting step comprises reducing the force at the other end of the strip when the pressure at the first mentioned end of the strip increases.
 4. A method as defined in claim 3, further comprising the step of terminating the application of force to the other end of the strip and increasing the force which acts against the first mentioned end of the strip.
 5. A method as defined in claim 1, wherein said adjusting step comprises maintaining the force acting against the end of the strip which is remote from the mine face at a constant value and increasing the force acting against the remainder of the strip when the result of the measurement indicates an increase in the pressure exerted by the strip.
 6. A method as defined in claim 1, wherein the variable force is transmitted by way of a roof engaging assembly which is subject to bending stresses and wherein said adjusting step comprises changing the force in the region of that end of the strip which is remote from the mine face when the pressure transmitted to the roof engaging assembly generates a bending stress which exceeds a predetermined value.
 7. A method as defined in claim 1, further comprising the step of supporting from below with a variable force at least one additional strip of the roof adjacent to the first mentioned strip, measuring the pressure which the additional strip exerts in a downward direction, and adjusting the variable force which acts on said first mentioned strip in accordance with the result of measurement of the pressure transmitted by the additional strip.
 8. A mobile mine roof supporting apparatus, particularly for use in longwall mining, comprising a support arranged to rest on the floor of an underground excavation; an elongated roof engaging assembly having front and rear portions and arranged to engage the roof in the excavation; at least one adjustable variable-lEngth propping device mounted on said support and connected with said assembly between said rear and front portions whereby the assembly is respectively urged against and moves away from the roof in response to lengthening and shortening of said device; at least one adjustable balancing device connected with and operative to apply to the rear portion of said assembly a force in line with the load upon said rear portion; and a control system comprising at least one signal generating load monitoring member responsive to changes in load upon said assembly and adjusting means for adjusting at least one of said devices in response to signals from said monitoring member.
 9. Apparatus as defined in claim 8, wherein said balancing device is connected with said support and is adjustable to exert a thrust or a pull on the rear portion of said assembly.
 10. Apparatus as defined in claim 9, wherein said balancing device comprises at least one hydraulically operated ram having a first portion coupled to the rear portion of said assembly and a second portion coupled to said support behind said propping device, one of said portions of said ram being movable lengthwise of the ram relative to the other portion of said ram and said adjusting means comprising means for moving said one portion of said ram in response to said signals.
 11. Apparatus as defined in claim 8, wherein said monitoring member comprises a pressure gauge mounted in the front portion of said assembly so as to be subjected to the pressure of the roof and conduit means for transmitting signals from said gauge to said adjusting means.
 12. Apparatus as defined in claim 8, wherein said monitoring member comprises a pressure gauge mounted on the rear portion of said assembly so as to be subjected to the pressure of the roof and conduit means for transmitting signals from said gauge to said adjusting means.
 13. Apparatus as defined in claim 12, wherein said one device is said balancing device and wherein said signals from said pressure gauge are indicative of maximum permissible bending stresses upon said assembly.
 14. Apparatus as defined in claim 8, wherein said balancing device comprises at least one hydraulically operated cylinder and piston unit of variable length and wherein said one device is said balancing device.
 15. Apparatus as defined in claim 14, wherein the cylinder of said unit is a double-acting cylinder.
 16. Apparatus as defined in claim 14, wherein said unit is a multi-stage cylinder and piston unit.
 17. Apparatus as defined in claim 8, wherein the front portion of said assembly comprises a cover plate which yieldably engages the roof and said monitoring member comprises a pressure gauge mounted in said front portion below said cover plate to produce signals in response to displacement of the cover plate by the roof.
 18. Apparatus as defined in claim 8, wherein the rear portion of said assembly has a downwardly and rearwardly inclined roof-engaging upper side and said balancing device comprises at least one hydraulically operated ram coupled to said rear portion and to said support and extending substantially at right angles to said upper side.
 19. Apparatus as defined in claim 8, further comprising means for pivotally connecting said assembly to the upper end of said propping device.
 20. Apparatus as defined in claim 8, further comprising a link connected to said support behind said devices and to the front portion of said assembly.
 21. Apparatus as defined in claim 20, wherein said support comprises a transversely extending rear wall having an upper portion located substantially midway between the upper side of said assembly and the underside of said support, and means articulately connecting the ends of said link to said front and upper portions.
 22. Apparatus as defined in claim 20, further comprising a second mine roof supporting apparatus having a floor-engaging support laterally adjacent to and connected with said first mentioned supporT said link being disposed in a vertical plane between said two apparatus and being further connected to the front portion of the roof engaging assembly of said second apparatus.
 23. Apparatus as defined in claim 8, wherein said roof engaging assembly comprises an elongated beam, means for pivotally connecting said beam to the upper end of said propping device, and at least one roof engaging cap reciprocably supported by said beam.
 24. Apparatus as defined in claim 23, wherein said cap is pivotable in a vertical plane with reference to said beam.
 25. Apparatus as defined in claim 23, wherein said balancing device comprises at least one ram mounted on the rear portion of said cap and having a portion movable upwardly against the roof.
 26. Apparatus as defined in claim 25, further comprising means for coupling said ram to said support.
 27. Apparatus as defined in claim 25, wherein the portion of said ram is shorter than said cap and is slidable along the roof.
 28. Apparatus as defined in claim 25, further comprising means for articulately connecting said portion of said ram to said cap.
 29. Apparatus as defined in claim 8, wherein said roof engaging assembly comprises an elongated beam connected to the upper end of said propping device and a pair of interconnected elongated roof engaging caps reciprocably by said beam.
 30. Apparatus as defined in claim 29, wherein said caps have front end portions connected to each other and wherein said monitoring member is supported by the front end portions of said caps.
 31. Apparatus as defined in claim 29, wherein said balancing device comprises two hydraulically operated rams each connected with the rear portion of one of said caps.
 32. Apparatus as defined in claim 8, wherein said roof engaging assembly comprises a pair of parallel elongated beams mounted on the upper end of said propping device and an elongated roof engaging cap disposed between and reciprocably supported by said beams.
 33. Apparatus as defined in claim 32, wherein said cap is pivotable in a vertical plane with reference to said beams.
 34. Apparatus as defined in claim 32, wherein said balancing device comprises a hydraulically operated ram connected with the rear portion of said cap. 